docs: def save(cls,instance,filename):
          """Class method save for saving TracingVariable."""
          filename = cls.correct_file_extension(filename)
          try:
              with open(filename,'wb') as f:
                  pickle.dump(instance,f,protocol=pickle.HIGHEST_PROTOCOL)
          except MemoryError as e:
              print ('{} occurred, will downsampled the saved file by 20.'
                     .format(type(e).__name__))
              copy_instance = instance.copy()
              copy_instance.H = copy_instance.H[::20,:,:]
              copy_instance.Y = copy_instance.Y[::20,:]
              with open(filename,'wb') as f:
                  pickle.dump(copy_instance,f,protocol=pickle.HIGHEST_PROTOCOL)